1. Field of the Invention
The present invention relates to a pulse radar based on the principle that a distance is computed from a time difference from when a pulse signal is reflected from a target until when the reflected pulse signal is received. Particularly, a pulse radar is mounted on an automobile and on an aircraft which flies horizontally at a very low altitude.
2. Description of the Related Art
The following three systems have hitherto been used for a radar and a radio sensor, as disclosed in “New Millimeter Wave Technique” pg. 278 (Tasuku TESHIROGI/Tsutomu YONEYAMA, Ohmsha Co., Nov. 25, 1999)
(1) Pulse Radar
(2) FM-CW Radar (Frequency Modulation Continuous Wave Radar
(3) Two-tone CW (Continuous Wave Radar)
In the “pulse radar” system (1), a signal which is shaped like a pulse from a high-frequency waveform is repetitively transmitted at a certain interval. The thus-transmitted pulse signal is scattered by a target. Then, the scattered wave is received as an answering wave. A time between end of transmission of the pulse signal and reception of the answering wave from the target becomes proportional to a distance from the target. Accordingly, in the “pulse radar” system, the distance to the target is measured on the basis of a period from the time transmission of the pulse signal is completed until the level of the answering wave from the target exceeds a threshold value. In the “FM-CW radar” system (2), a transmission signal is frequency-modulated at a given period and whose frequency is iteratively increased and decreased at a given ratio. The transmitted frequency-modulated signal is scattered by a target, and the scattered wave is received as an answering wave. The receiving end extracts a difference (hereinafter described as a “beat frequency”) between the frequency of the transmission wave and the frequency of the received wave. The beat frequency becomes proportional to a distance from the target. As mentioned above, the “FM-CW radar” system measures a distance by detecting the beat frequency of the signals. Moreover, the two-tone CW system (3) is a system which transmits two waves (which cause a phase difference as a distance changes) slightly differing from each other in terms of a frequency, such as f1=24 GHz and f2=24 GHz+75 MHz. A distance is measured by detecting a phase difference between two waves arising within a roundtrip propagation time to the target.
Prior arts for reducing interference by the transmission pulse signal to the receiving signal have been proposed. JP-A-05-56009 discloses a transceiver which, when simultaneously performing transmission and receipt of a pulse signal, removes an impediment by interference of the transmitting end to the receiving end, without distortion of a received wave signal. Specifically, the transceiver has an adaptive filter and a subtractor as interference wave suppression means. The adaptive filter simulates a received interference wave signal by the transmitting ends to the receiving end, by inputting a part of a transmission signal additionally provided with a broadband signal of the transmitting end. The subtractor subtracts a signal output of the adaptive filter from a signal output of the receiver. JP-A-2003-279649 discloses hindering interference of a transmission wave by interposing a flat shielding plate between a transmission antenna and a receiving antenna.
A short-range radar demands short-range resolution rather than the maximum detectable range. For instance, a short-range radar for use with an automobile driving assist system is intended such as a parking assist, object detection in a blind spot, a pre-crash alarm, a sideways proximity alarm, and the like. Although a maximum detection range of about 3 m is allowed in many cases, a minimum distance resolution is desired less than 10 cm. However, when a short-range radar is implemented by the systems (1) through (3), any of the systems involves broadening of an occupied bandwidth with varying degrees. Broadening of the occupied bandwidth raises various problems, such as a necessity for a high frequency for a carrier wave, difficulty in designing of an antenna, and an increase in cost of a circuit, as well as a decrease in the number of channels which can be set in a single frequency band.